Cathode Bypass Switch thump again!!!

[deuce42]

hi guys

Apologies as I know this has been covered many times but in previous threads I still seem to not be getting the point!!!

I'm trying to use a switch to flick between 2.7k/0.68uf and 1.5k/22uf on my v1 cathode. Loud familiar bang when using the switch.

As per my diagram below, I understood from other posts that I could add a 1m resistor between the cathode of v1 and ground since the middle leg of the switch also goes to ground. This 1m resistor trick isn't stopping the pop.

Can someone explain what I'm doing wrong?

Apologies if I'm revisiting old territory.

[darryl_h]

[IMG:595:608]http://i497.photobucket.com/albums/rr333/valvetone/2014%20Projects/cathode_zps5a3d5a52.jpg[/img]

This may not completely eliminate the thump, but it should reduce it.

The 2.7k/0.68uF pair remain permanently attached, and the second resistor/capacitor set are switched in or out. The 22uF completely swamps the .68uF, so both in parallel is still very close to 22uF. The 3.3k resistor in parallel with the 2.7k is near as dammit to 1.5k.

The 47k resistor charges the 22uF capacitor to cathode potential to minimise thumps, although the cathode voltage will change as the 3.3k resistor is switched in.

[deuce42]

Ok thanks Darryl

Just tried it. Still a bit of a thump when I flip the switch but it doesn't sound like a bomb has gone off behind me like it did before.

Cheers
Rich

[roberto]

If you switch from one Rk to another you can't eliminate the pop, just because you are istantaneously changing the bias, so the voltage.

You can eliminate the pop due to different Cks, by the way suggested in the previous post.

[Stevem]

How about a make before break type switch?

[roberto]

Same problem, as the Rk changes, the current of that stages changes, so the voltages at cathode and plate, so here comes the BUMP.

[Structo]

This is what I did to my D clone.
Switch on gives 5.38uF for single coil, switch off gives .68uF for humbucker.

It doesn't thump.

[martin manning]

Combining the suggestions above, try this:

[Smokebreak]

Martin, can you explain the effect of the 27K in series, and how it seemingly nullifies the 3K3//22u, when in 2K7//.68 mode? How does one arrive at that value? I'm seeing 27K, 47K, 4K7 in the drawings

[bruce egnater]

I think no matter what you do, you may get some pop because of the abrupt bias shift. I would suggest trying Structo's method. Yes, you don't get the exact DC bias current as you would with changing resistors but the AC LF response will change and be very close to what you want with maybe no pop.

[ampfab]

why not just have a 2.7k resistor and parallel in the 22uf cap with a switch? with a 22uf cathode bypass cap can you hear a difference between 2.7k and 1.5k?

[martin manning]

Jeremy, I wanted to make the resistor just big enough to keep the impedance to ground through the 22u around 5 times the impedance of the 0.68u down to the bottom of the audible frequency range.

Ampfab, that's a big shift in the bias point from 1k5 to. 2k7.

[roberto]

I think it will pop due to the fact that the cap is at cathode voltage from one side and around 9/10th of the cathode voltage on the other side (due to the 3k3+27k voltage divider), that is then abruptly grounded by the switch.

[Merlinb]

Roberto is right, you'll always get a thump if you try to change RK (and therefore bias voltage) on the fly. The only way around it would be to change Rk veeerrryyy sloooowwwwly.
e.g. use an LDR and an LED with that turns on over a couple of seconds (using an RC charging circuit or something).